This invention relates to the sealing of overlapped rubbery olefinic polymer (i.e., homopolymer, copolymer, terpolymer, etc.) sheets to each other, to spliceable sheets, to a method of making such sheets, and to a transfer tape having particular utility in the manufacture of the heat-sealable sheets.
Rubbery olefinic polymer sheet material finds widespread industrial use in applications where it is necessary to contain or exclude liquids. Compared to vinyl sheet material, the rubbery olefinic polymer sheet materials have longer life, greater flexibility and resilience at low temperatures, ability to withstand high temperatures without stretching or softening unduly, and superior resistance to ultraviolet light. The most widely used rubbers for formulating these sheets are polymers of ethylene, propylene, and diene monomers (commonly known as EPDM), butyl rubber, and blends of the two. In making rubbery sheet material, the olefinic polymers are commonly blended with desired fillers, coloring agents, extenders, vulcanizing or crosslinking agents, antioxidants, etc. to form a "compound", which is then calendered or extruded into sheets (commonly known as "membranes"), typically on the order of 1.5 millimeters thick and 2 to 6 meters wide. These membranes are then heated to perhaps 150.degree. C. for 2 hours to effect vulcanization.
For many of the applications in which rubbery membranes are employed, it is necessary to overlap and splice the edges of a large number of sheets. Rubbers, which have low energy surfaces, are unreceptive to many adhesives. Accordingly the splicing procedure has heretofore typically involved the steps of cleaning the overlapped areas, applying a primer solution to each face, allowing the solvent to evaporate, applying a contact adhesive solution to each face, again allowing the solvent to evaporate to leave a tacky adhesive surface, mating the contact adhesive-coated surfaces, and compacting the spliced area with a heavy steel roller. Operating in this way, it is possible to form extremely large spliced membranes which can be used to line water reservoirs, irrigation canals, sewage lagoons, industrial waste pits, and solar energy ponds; such products are popularly designated "geomembranes". The splicing process is, however, extremely labor-intensive and costly.
Another important application for spliced rubbery membranes is in the installation of flat roofs for commercial, institutional, and industrial buildings. Spliced membranes are laid over new or existing roofs and typically either fastened down (e.g., at 40-centimeter intervals) with metal battens or ballasted with round river-washed stones. When employed on a roof, extremely rigorous demands are placed on the rubbery membranes, particularly on the splices. Roof temperatures may approach the boiling point of water when exposed to the summer sun, and they may sink to -30.degree. C.--or even lower--during the winter.
In all of the applications for spliced membranes just described, the splicing has been a tedious and time-consuming part of fabrication. How to simplify the preparation of lapped splices has remained an unsolved problem.